TEXT-BOOK OF BACTERIOLOGY. 23 



of filtration, and which then causes this change in the gelatin 

 alreadj' mentioned. 



We have the bacteriaj then, as the buriers of the organic world; 

 it is their task to put out of the way the material that has become 

 useless, and thus to make room for new living forms. 



Yet, as we already know, they by no means restrict their at- 

 tacks to dead and useless things; they also penetrate into living 

 organisms, grow and multiply in them, and thus, as they can only 

 do so at the cost and to the injury of the individual attacked, they 

 become the source of quite a number of the most varied pathologi- 

 cal phenomena. The diseases known to be caused by some partic- 

 ular species of bacteria are becoming more and more numerous, 

 and as their origin is external they are called infectious diseases. 



In a general way we distinguish the bacteria which possess 

 these "infectious" qualities as pathogenic, and those which are 

 harmless and cannot produce these effects in other organisms as 

 non-pathogenic. 



Besides these three chief fields of bacterial activity — the excita- 

 tion of fermentation, of putrefaction, and of infection — there are 

 others of minor importance. 



That which most strikes the eye is the formation of coloring 

 matter by a number of species, most of them harmless. All kinds 

 of colors may be observed : white, black, blue, green, brown, red, 

 orange, etc., some of them of the brightest hue. How the forma- 

 tion of coloring matter is accomplished is not yet known with cer- 

 tainty. Probably the majority of micro-organisms do not generate 

 the pigment directly, but only the basis of it — a chromogenic body. 



If this is liberated in any way, for instance by its passing 

 through the membrane of the cell by diffusion, or by the death and 

 decomposition of the micro-organisms, it has an opportunity to 

 combine with certain ingredients of the culture medium, or to gain 

 access to the oxygen of the air, and then, but not till then, does the 

 color appear. This explains why the pigment is often observed 

 only on the surface of our cultures, and why the tiit is, as a rule, 

 dependent on the nature of the substratum. 



There is another remarkable property of some few bacteria, es- 

 pecially investigated by Fischer, which " phosphoresce " or shine in 

 the dark. They do so, under some circumstances, with such a de- 

 gree of brilliancy that, by the light of a few gelatin cultures, the 

 time may be read from the face of a watch — nay, it has even been 

 possible to photograph such cultures by the light which they them- 

 selves emitted. 



